1. Technical Field
The present invention relates to a device for preventing overfilling of liquid storage tanks and, more particularly, to a float-activated shut-off valve.
2. Discussion of the Prior Art
Overfilling of fuel storage tanks, for example gasoline tanks used at filling or service stations, is recognized as a major source of pollution and a loss of a valuable resource. As a consequence, federal law now requires that existing underground storage tanks be fitted with automatic shut-off devices by Dec. 22, 1998. Shut-off valves designed to automatically cut-off inflow into tanks before overfilling occurs are being introduced in increasing numbers in anticipation of the new regulations.
Overfilling can be avoided by manually shutting off the flow at the appropriate time. Because operators cannot timely gauge overflows and are susceptible to errors, reliance on manual shut-off is inefficient and uncertain.
Automatic shut-off valves typically employ a float to actuate a valve, which in turn blocks the inflow of the liquid when the level in the tank rises to a predetermined level. They are designed to fit through the opening used to introduce liquid into the tank. Most existing fuel storage tanks have liquid supply sleeves on the order of about four inches in diameter located on the tank top. The need to retrofit existing tanks therefore dictates the need for float sizes which are limited in size and shape, placing significant restrictions on valve design.
A drop tube assembly, described in U.S. Pat. No. 5,235,999 (Lindquist et al), includes a float coaxially located with the drop tube, an elongate conduit extending from the surface to a position near the tank bottom. This float moves up and down along the drop tube, actuating the valve. Such a design significantly restricts the volume of the float of a given length, as the float is limited by the diameter of the opening in the tank and the diameter of the drop tube. Also, inasmuch as the float remains parallel to the valve assembly along the drop tube, it exerts no leverage on the valve, and as a result, insufficient force is generated to completely shut off high flow rates.
Another commercially available valve, the Stopper, manufactured by OPW Fueling Components of Cincinnati, Ohio, uses an in-line cylindrical float that controls a release mechanism to initiate an abrupt valve closure with the hazard of potentially damaging shock or hammer effects.
An alternative design, manufactured by the Clay & Bailey Manufacturing Company of Kansas City, Mo., the MINO Filling Limiter, utilizes a float which fits around half the circumference of the drop tube. The float is attached to the valve by a lever arm. As the float rises, it rotates a ball in the flow conduit. The ball has a through-hole passage defined in one direction allowing fluid flow to pass while the through-hole aligns at least partially with the conduit. The position of the float in the tank regulates the amount of flow through the valve by changing the alignment of the through-hole in a manner that stops the flow when the float assumes a horizontal position and the tank is full. Although the float is attached to a lever arm, the shape of the float is defined by the drop tube, and, in order to accommodate the lever arm, the float cannot occupy the entire cross-sectional area of the drop tube. These restrictions on size, shape, and volume limit the leverage the float can exert on the rotating valve. Major disadvantages of this design are that incremental flow restriction begins as soon as float movement is initiated, increasing the time required to fill the tank and that limitations on the shape of the float frequently preclude use in small tanks because of insufficient clearance.